1. Field of the Invention
The present invention relates generally to a scanning system driving device used in an image forming apparatus, having a scanning system for illuminating and scanning a document, such as a copying machine or an image scanner, and more particularly, to an improvement in transmission control of a driving force of the scanning system driving device.
As a scanning system provided for a copying machine or the like, there are two types of scanning systems: a scanning system in which an optical system is moved relative to a still document, and a scanning system in which a document platen is moved relative to a still optical system. In this specification, the term "scanning system" is used as a general term for the above described two types of scanning systems. Consequently, a scanning system driving device includes both a device for moving and controlling an optical system and a device for moving and controlling a document platen.
2. Description of the Prior Art
An image forming apparatus such as a copying machine includes a scanning system for optically scanning a document by driving an optical system or a document platen. For example, in a copying machine comprising a scanning system in which an optical system is driven, a moving body holding a light source and a reflecting mirror is displaced at uniform speed relative to a document. Consequently, the surface of the document is illuminated and scanned. In the process, a photoreceptor is exposed by light reflected from the document, and an electrostatic latent image is formed on the surface of the photoreceptor. This electrostatic latent image is developed into a toner image, and the toner image is transferred and fixed to copy paper, to achieve copying.
In such an image forming apparatus such as a copying machine, there are strong demands toward improvements in the image quality, along with miniaturization and higher speed. Therefore, improvements have been made for shortening the rise time elapsed before the scanning speed of the document by the scanning system reaches a predetermined speed, as well as preventing the nonuniformity of speed from occurring during the scanning of the document.
The electrostatic latent image formed on the surface of the photoreceptor is formed by the light reflected from the document illuminated and scanned. In order to increase the copying speed, therefore, it is essential that the scanning system be moved at high speed. Furthermore, in order to accurately reproduce a document image, it is necessary to keep the scanning speed constant.
On the other hand, if the copying machine is miniaturized, the moving distance of the scanning system from the home position to the position where illumination and scanning are started is short. Therefore, it is necessary to rapidly increase the moving speed of the scanning system. If the moving speed of the scanning system is thus rapidly increased so-called overshoot is produced. That is, the speed of the scanning system once exceeds the rated speed immediately before the speed of the scanning system reaches the rated speed. This overshoot degrades the copy image corresponding to the region in the vicinity of the position where illumination and scanning are started. Consequently, care must be taken to exclude the effect of the overshoot.
A prior art technique in which the moving speed of a scanning system is rapidly increased and a copy image is not affected by overshoot is shown in FIG. 11. A driving force on the scanning system is supplied from a main motor, provided in the copying machine, through an electromagnetic clutch. FIG. 11(a) shows the change with time of the voltage applied to the electromagnetic clutch, and FIG. 11(b) shows the change with time of the moving speed of the scanning system.
In this prior art technique, in order to rapidly increase the moving speed of the scanning system, the voltage applied to the electromagnetic clutch is instantaneously increased to a predetermined rated voltage at the time of starting the scanning system, as shown in FIG. 11(a). This predetermined rated voltage is a voltage applied to the electromagnetic clutch when the scanning system is driven at rated speed so as to illuminate and scan a document. In FIG. 11, an idle period is a period elapsed from the time when the rotation of the main motor is started until the electromagnetic clutch is connected, an approach period is a period elapsed from the time when the scanning system is in the home position until it reaches the position where illumination and scanning are started, and an image forming period is a period during which the document is illuminateded and scanned.
In this prior art technique the electromagnetic clutch is instantaneously connected, so that the driving force from the main motor is impulsively transmitted to the scanning system. Immediately after the scanning system rises, therefore, the moving speed of the scanning system fluctuates, as shown in FIG. 11(b). If the moving speed vibrates, excellent image formation is not carried out. Consequently, a period during which the moving speed vibrates is taken as an approach period, and a period after the moving speed is stabilized is taken as an image forming period.
In this prior art technique, however, it is necessary to ensure a sufficient approach period to stabilize the moving speed of the scanning system. Therefore, the distance from the home position to the position where illumination and scanning are started of the scanning system, that is, an approach distance, is increased. Therefore, there occurs a problem that the copying machine is prevented from being miniaturized.
This problem is solved by another prior art technique shown in FIG. 12. In this prior art technique a applied to an electromagnetic clutch is gradually increased, as shown in FIG. 12(a), Therefore, a driving force from a main motor is gently transmitted to a scanning system. As a result, the moving speed of the scanning system is smoothly increased and stabilized at rated speed in a short time, as shown in FIG. 12(b). Consequently, no long approach distance is required, thereby making it possible to miniaturize the copying machine, unlike the prior art technique shown in FIG. 11.
In this prior art technique however, a dedicated electric circuit of complicated construction is required to gradually increase the voltage applied to the electromagnetic clutch. Therefore, there occurs a new problem in that the cost is increased.
Still another prior art technique by which this problem is solved is disclosed in Japanese Patent Laid-Open Gazette No. 148138/1986. This gazette discloses the technique for applying a pulse voltage to an electromagnetic clutch in the early stages of scanning and repeatedly connecting and/or disconnecting the clutch to achieve a substantially half clutched state. In this prior art technique, however, the pulse period, the pulse content and the like of the pulse voltage are not sufficiently considered. That is, the natural vibration of an optical system is promoted by repeatedly connecting and/or disconnecting the clutch, so that the image quality is liable to be degraded.